The present invention relates to a drive device for a a motor vehicle axle and, in particular, to a drive device for an electrically driven axle of a motor vehicle.
It is generally known, in the case of electrically driven motor vehicles or in the case of hybrid drives, for one or both axles of the motor vehicle to be driven by way of an electric machine. The electric machine outputs a drive torque via a transmission gearing to a downstream differential. To cover the speed range of the motor vehicle with manageable rotational speeds at the electric machine, it may be advantageous for this purpose for the transmission gearing to be switchable into two transmission ratio stages.
A problem addressed by the invention is that of proposing a drive device of the generic type which, while being of structurally compact construction, permits expedient torque distributions and toothing engagement ratios which are particularly well adapted to electric machines.
The solution to said problem, according to the invention, provides that the transmission gearing is formed by two planetary gear sets which are coupled to one another and which can be switched into two transmission ratio stages by way of switching elements. The transmission gearing, which is in the form of a 4-shaft coupling gearing, may be of a nested design in an advantageous manner in terms of gearing technology, and furthermore, owing to the numerous possible tooth engagements, permits the transmission of high levels of power while being of structurally compact construction.
Here, it is particularly preferably possible for the switching elements to be formed by two brakes, which each immobilize at least one gear element of the planetary gear sets. The brakes, which are positioned in the gear set housing, are operable (electrically or hydraulically) more easily from a construction aspect than clutches, and furthermore effect partial support of the drive torques directly into the gear set housing while simultaneously reducing the load on the respective gear elements and/or switching elements.
In one advantageous refinement of the invention, the planetary gear sets may, in their stationary transmission ratio, be in the form of minus gear sets. Minus gear sets have, inter alia, the advantage that they can be of structurally simple design and require little structural space in terms of their radial extent.
In a preferred embodiment of the invention, the driven element of the planetary gear set may be the sun gear of the first planetary gear set, and the drive output element may be the web of the first planetary gear set, wherein furthermore, the web of the first planetary gear set is coupled to the internal gear of the second planetary gear set, and the internal gear of the first planetary gear set is connected to the web of the second planetary gear set, and wherein one switching element is connected in terms of drive to the web, and the other switching element is connected in terms of drive to the sun gear of the second planetary gear set. With this coupling of the two planetary gear sets, it is the case in the first transmission ratio stage that the drive torque from the electric machine runs only via the first planetary gear set, while the second planetary gear set is free from load. By contrast, in the second transmission ratio stage, all gear elements or shafts of the planetary gear sets transmit power, and are accordingly subjected to load. By changing the transmission ratio of the second planetary gear set, it is possible for the step change between the transmission ratio stages and the transmission ratio in the second transmission ratio stage to be adapted, without significant modification of the gear set construction, to different levels of drive power of the electric machine and/or to different vehicle types, for example.
Here, it is possible in a particularly advantageous manner in terms of design and construction for a web cheek of the web of the first planetary gear set to form a structural unit with the internal gear of the second planetary gear set. This structural unit may be of relatively solid form, whereby the distortion that often occurs in the manufacture of internal gears can be eliminated and high levels of concentricity and toothing quality can be realized. This ensures improved running smoothness at the high rotational speeds that are encountered in the case of electric drives and at the resulting high toothing engagement frequencies.
Furthermore, the web cheeks, which face toward one another, of the first and of the second planetary gear set may be arranged radially one above the other (in nested form) and substantially in the same plane of rotation. This results in a significant axial shortening of the drive device, which furthermore promotes flexurally rigid mounting of the gear elements in the gear set housing.
Furthermore, it is particularly advantageous if the two switching elements or brakes are arranged radially one above the other and may therefore be standard components such as are known for example from dual-clutch transmissions (in this case in the form of clutches).
In a further advantageous refinement of the invention, the internal gear of the second planetary gear set may have an external toothing which, in conjunction with a pawl mounted pivotably in the housing, acts as a parking lock. As a result of the specified coupling of the two planetary gear sets, said internal gear forms a fixed connection, in terms of gearing, to the output shaft to the differential, such that, as is conventional in the case of parking locks, the drive wheels of the motor vehicle are blocked via the gear elements, regardless of the state of the switching elements, when the internal gear is blocked.
In a highly advantageous refinement of the invention, a transmission drive mechanism with step-down transmission ratio may be connected into the force flux between the planetary gear sets and the differential, which transmission drive mechanism raises the rotational speed level in the planetary gear sets, whereby the gear elements and switching elements thereof can be designed to be of a more lightweight construction despite high drive output torques at the wheels of the motor vehicle.
The transmission drive mechanism may particularly preferably be formed by a sun gear connected to the drive output web of the first planetary gear set, by a web which bears planet gears and which outputs drive to the differential, and by an internal gear fixed with respect to the housing. Owing to the support of the internal gear in the housing, the loads on the switching elements and brakes are reduced to an even greater extent.
In an embodiment of the mounting of the planetary gear sets in the housing of the drive device, which is particularly advantageous in terms of construction, the web of the first planetary gear set and the internal gear of the second planetary gear set may be mounted, by way of a common annular flange and a rolling bearing, on a drive shaft which is connected to the electric machine and which bears the sun gear and which has a radially outwardly extending bearing ring, or directly in the gear set housing. Furthermore, the web of the second planetary gear set may be supported, together with the internal gear of the first planetary gear set, in the annular flange by way of a further rolling bearing. Finally, the web of the second planetary gear set may additionally be rotatably mounted, by way of a third rolling bearing, on a hollow shaft which connects the sun gear of the second planetary gear set to one switching element or to one brake. These features result in the gear elements being mounted in a flexurally rigid fashion, and so as to be capable of withstanding high rotational speeds, with a low outlay in terms of bearing technology.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.